1. Field
This invention relates to and has as its principal objective improved particles of electrostatic development powders, often referred to as "toners", which are preferably spheres with a diameter of 5 to 50 microns and which consist of a binder in form of a high molecular weight organic substance which contains finely divided particles of ferromagnetic material.
2. State of the Art
Electrostatic development powders are used for the development of latent electrostatic images which have been generated through image-wise discharge by electromagnetic radiation of electrostatically charged organic or inorganic photoconductors.
The use of development powders which contain dispersed ferromagnetic material, usually magnetic iron oxide Fe.sub.3 O.sub.4 (magnetite) is disclosed in U.S. Pat. No. 2,874,063, wherein it is taught that images are preferably developed by application of those powders utilizing a development station which comprises a cylinder of nonmagnetizable, magnetically transparent material (e.g. aluminum) which encloses magnetic rolls. The cylinder carries a layer of toner particles which are kept in place by magnetic forces. Such a development station is often referred to as "magnetic brush."
For the development of the latent electrostatic images, the photoconductor layer is transported past the development station and an electrostatic force is exerted upon the toner particles. The electrostatic force acts in the opposite direction to the magnetic attraction force associated with the cylinder which carries the toner particles. The toner particles are selectively transferred from the cylinder to the photoconductor surface at these locations where the electrostatic force is larger than the magnetic force. The toner can then be fixed upon the photoconductor surface by application of pressure or a combination of heat and pressure. Alternatively, the toner can be transferred by the xerographic process from the photoconductor to a sheet of normal paper and then fixed on the paper by application of pressure or a combination of pressure and heat.
U.S. Pat. Nos. 4,246,331, 4,145,300 and 4,134,676 teach that finely dispersed particles of dyestuff which are volatile at atmospheric pressure and temperatures between 100.degree. C. and 220.degree. C. can be incorporated in the toner particles. By development of latent electrostatic images with development powders containing such a dyestuff, single color transfer papers can be produced. The single color transfer papers can be used for heat transfer printing of various items and, in particular, of polyester fabrics. As described in French Pat. Nos. 1,233,330 and 1,585,119, the transfer paper is brought into contact with a polyester fabric, and through the application of heat and pressure, the dyestuff evaporates and is transferred from the transfer paper to the fabric on which a single color reproduction of the original is formed. This process leads to acceptable results for flat and lightweight woven fabrics. It would, however, be highly desirable to increase the pentration of the print into the fabric to avoid the "grinning" phenomenon, i.e. the sight of the white, uncolored depth of the fabric when it is stretched.
Penetration of the dyestuff is generally insufficient when pile structures like carpets are heat transfer printed with the color transfer paper because only the very top of the pile structure is colored by the transfer process. For applications requiring deeper penetration, special "deep penetrating" dyestuffs have been proposed. Typical dyes which are representative of the deep penetrative types are the red dyestuffs 1-amino-2-chlor-4-hydroxyanthraquinone, 1-amino-2-brom-4-hydroxyanthraquinone, the blue dyestuff 1,4-diisopropylanthraquinone, and the yellow dyestuff having the formula ##STR1##
These special dyestuffs show good penetration and acceptable fastness values for heat transfer printing of polyamide 66, the standard fiber material for carpets, but the lightfastness values of the dyestuffs when used on polyester materials are poor.
The molecules of the highly volatile dyestuffs used in making the color transfer papers are quite mobile even at room temperature and for the toners described in U.S. Pat. Nos. 4,246,331, 4,145,300 and 4,134,676, the dyestuffs migrate on storage at room temperature to the surface of the toner particles. This is a distinct disadvantage inasmuch as the dyestuff at the surface of the toner particles tends to pollute the magnetic brush, which, in turn, results in an unselective, undesirable deposit of dyestuff on the surface of the photoconductor. This results in a commercially unacceptable transfer in which the printed article generally has an unacceptable, nonuniform colored background.
Problems have also been encountered with transfer papers utilizing conventional toners when the transfer papers are used in heat transfer printing of anodized aluminum. U.S. Pat. No. 3,484,342 teaches that anodically oxidized aluminum can be heat transfer printed as long as the pores of the oxide film are unsealed. Transfer papers carrying one or several sublimable dyestuffs are brought into contact with the oxide covered aluminum surface, and the sandwich is treated for 30 to 60 seconds at a temperature of from 180.degree. C. to 210.degree. C. The volatilized dyestuff is transferred by diffusion or absorption into the oxide film on the anodized aluminum. The dyestuff can be fixed in the pores of the oxide film by several known methods including sealing with boiling water. This heat transfer printing process can also be performed with papers produced by the electro-photographic process with conventional toners. However for this application by far the most important shade is black, and black images are only poorly reproduced, with shades of gray being obtained because of the limited dyestuff concentration in the toner particles.
Particularly good results with the electro-photographic process are achieved with a combination of the dyestuffs C.I. Disperse Yellow 54 (yellow), C.I. Vat Red 41 (magneta), C.I. Disperse Red 60 (bordeaux), C.I. Disperse Blue 331 (cyan), C.I. Disperse Violet 23 (blue), and C.I. Disperse Violet 27 (blue).
Another example for the technological and commercial importance of high intensity heat transfer prints is furnished by the printing of natural silk fabrics. The Japanese Patent Publication No. 53-78,366 teaches that silk fabrics can be heat transfer printed if the fibers are swollen with water and if the swollen state is maintained after drying by use of high molecular polydiols such as polyethylene glycol 300. The swollen state of the silk fibers can also be stabilized with the boric acid ester of a 1 to 6-valent alcohol which is commercially available under the trade name Glyecin CD (BASF). The fabric prepared in this manner can be heat transfer printed with conventional transfer printing dyestuffs, and after the swelling stabilizer has been removed by rinsing with water or a solvent, prints with reasonable color transfer and light fastness values are obtained. Particularly good results are achieved with dyestuffs comprising C.I. Disperse Yellow 54, C.I. Vat Red 41, C.I. Disperse Red 60, C.I. Solvent Blue 63, and C.I. Disperse Blue 332. Traditionally, however, silk prints demand bright colors and the conventional toner gives rather ineffective dull shades only. This is especialy so with black toners which yield a shade of dark brown. Toners which produce brighter colors would, of course, be highly desirable.